How to make a ball capped screw?
You can try to start with lower resolution. Like here is use Circle with 12 vertices. Screw mmodifier with 12 Steps, Screw 1.73.
Duplicate Circle and use as top sphere, just change Screw to zero.

I'm not sure how transition should looks like, but you can just use Boolean modifier &gt; Union ...
Before boolean, Apply Screw modifier, Merge by Distance 0.04 small inaccuracy. From top view box-select all center vertices and delete. So you get clean topology. Apply Boolean modifier. Loop select inner screw and mark as Sharp ⇧ Shift+E 1. Then add Subsurface modifier, Smooth shading. Under Data properties &gt; Normals &gt; enable Auto Smooth.

... or probably more natural it will result with upper part of sphere and manually delete unwanted faces and reconnect.